A CR oscillator is known, in which, among series-connected two-stage inverters, a resistor and a capacitor are connected between input node and output node of the latter-stage inverter, and a connection node of the resistor and the capacitor is connected to an input node of the former-stage inverter.
This type of CR oscillator is unique in excellent responsiveness even having a simple circuit configuration. However, since an input voltage of the first-stage inverter instantaneously becomes a high voltage that exceeds a power supply voltage, the former-stage inverter is required to withstand high voltages. Nevertheless, a high-voltage withstanding inverter has a longer propagation delay time than a regular inverter, and hence has a problem of reduction in oscillation frequency.
As explained above, the known CR oscillator configured with a plurality of stages of inverters, a resistor, and a capacitor can basically control the oscillation frequency based on a resistance value of the resistor and capacitance of the capacitor. However, since the first-stage inverter is required to withstand high voltages, the oscillation frequency is restricted by a propagation delay time of the first-stage inverter.